1. The Field of the Invention
The present invention concerns polymeric materials for use in non-linear optics. More specifically, it concerns a new type of amorphic polymers which have a group (chromaphore) in their side chains which, in order to give rise to an active material in non-linear optics, must have been oriented in an electrical field at a temperature close to that of glass transition, the performances of these polymers being particularly stable in time. It would appear to be very useful to use such polymers in the manufacture of components for integrated optics such as a frequency doubler operating at wavelengths of between 0.8 and 2 microns, an electro-optical modulator using an electromagnetic wave with a wavelength of between 0.6 and 2 microns.
2. Description of the Prior Art
Organic materials have shown that they provide non-linear optical performance which is at least as satisfactory as that of inorganic materials such as lithium niobate, potassium dihydrogen phosphate or gallium arsenide and, more specifically, polymers which offer major advantages in application, cost and range of structures as a result of molecular engineering, hold out considerable interest. To date, the polymeric materials available for the preparation of films for non-linear optics are as follows:
Solid solutions of small molecules active in non-linear optics (known as colorant) dissolved in an amorphic polymer or in a liquid crystal polymer.
Copolymers in which the active molecule is bonded to the polymer skeleton in order to avoid problems of solubility and thus increase the level of the entity responsible for non-linear optical effects. In this type of material, the dye attached to the polymer can no longer diffuse within the material but may be the site of relaxation after orientation which contribute to ageing of the performance of the material.
To offset these relaxation-related problems, various research teams have synthesized copolymers presenting a reticulable side chain or a chain carrying an entity which gives rise to non-linear optical effects. (P. Le Barny FR 88 05790) (Joowon Park and Tobin, J. Marks, Jian Yang and George K. Wong, Chem-Maten, 2, 229-231 (1990)). In this case, the main chains of the polymers can no longer move relative to one another as a result of the reticulation, but the side chains carrying colorant molecules retain a sufficient degree of freedom to relax and in time partially lose the orientation initially induced.
Another solution has also been envisaged, that of elaborating active polymers from small multi-function molecules which are active in non-linear optics and can be reticulated by heat treatment (M. Eich et al. (IBM), J. Appl. Phys., 66 (7) (1989)) (D. Jungbauer, B. Reck, R. Tweig, D. Y. Yoon, C. G. Willson and J. D. Swalen, Appl. Phys. Letter, 56 (26) (1990)). The main advantage of such systems lies in the high degree of stability obtained, since the chromophores directly constitute anchor points within the mesh. However, the preparation of the polymer remains vague, and the preparation of oriented films from liquid solutions containing small molecules with very low viscosity still requires the development of a deposition protocol and particularly difficult cooking cycles. Furthermore, the molecules most appropriate for the preparation of polymers of this type do not appear to be those most valuable in non-linear optics.